Stop using deprecated `backend` argument in `cryptography`

Per

- https://cryptography.io/en/latest/changelog/#v36-0-0
- https://cryptography.io/en/latest/faq/#faq-missing-backend

> `cryptography` stopped requiring the use of `backend` arguments in version 3.1 and deprecated their use in version 36.0. If you are on an older version that requires these arguments please view the appropriate documentation version or upgrade to the latest release.
>
> Note that for forward compatibility `backend` is still silently accepted by functions that previously required it, but it is ignored and no longer documented.

A lot of code in Azure CLI (mainly `keyvault` module) still uses `backend` argument in `cryptography`:

https://github.com/Azure/azure-cli/blob/47823b5782343aa9c6dd146a64b6f21c36ff5bb0/src/azure-cli/azure/cli/command_modules/keyvault/custom.py#L1324

https://github.com/Azure/azure-cli/blob/47823b5782343aa9c6dd146a64b6f21c36ff5bb0/src/azure-cli/azure/cli/command_modules/keyvault/custom.py#L1414

The best practice is to bump `cryptography` to newer versions (>=3.1):

https://github.com/Azure/azure-cli/blob/fbe0addc214c857fc67d89266aede50c21970705/src/azure-cli-core/setup.py#L49

and stop using deprecated `backend` argument, just in case `cryptography` totally removes `backend` one day.

**Additional context**
Related issue in Azure Python SDK repo: https://github.com/Azure/azure-sdk-for-python/issues/36579